Abstract

The antibacterial effect of the soybean phytoalexin glyceollin was assayed using a liquid microculture
technique. Log-phase cells of Bradyrhizobium japonicum and Sinorhizobium fredii were sensitive to glyceollin.
As revealed by growth rates and survival tests, these species were able to tolerate glyceollin after adaptation.
Incubation in low concentrations of the isoflavones genistein and daidzein induced resistance to potentially
bactericidal concentrations of glyceollin. This inducible resistance is not due to degradation or detoxification of
the phytoalexin. The inducible resistance could be detected in B. japonicum 110spc4 and 61A101, representing
the two taxonomically divergent groups of this species, as well as in S. fredii HH103, suggesting that this trait
is a feature of all soybean-nodulating rhizobia. Glyceollin resistance was also inducible in a nodDlD2YABC
deletion mutant of B. japonicum 110spc4, suggesting that there exists another recognition site for flavonoids
besides the nodD genes identified so far. Exudate preparations from roots infected with Phytophthora
megasperma f. sp. glycinea exhibited a strong bactericidal effect toward glyceollin-sensitive cells of B.
japonicum. This killing effect was not solely due to glyceollin since purified glyceollin at concentrations similar
to those present in exudate preparations had a much lower toxicity. However, glyceollin-resistant cells were
also more resistant to exudate preparations than glyceollin-sensitive cells. Isoflavonoid-inducible resistance
must therefore be ascribed an important role for survival of rhizobia in the rhizosphere of soybean roots.